1. Field of the Invention
The invention relates to antennas and more particularly to tunable antennas, especially for, but not limited to, vehicles.
2. Description of the Related Art
Most commercial trucks carry one or more antennas, the most common being AM/FM and citizen""s band (CB) antennas. The wide variety of vehicles and the number of different types of antennas available aggravate the problem of optimizing the performance of a given antenna on a given vehicle. Tuning is especially important for transmitting antennas such as CB. Ideally, each antenna will be tuned to the vehicle on which it is installed, but the requirement for specialized knowledge of antenna tuning and the labor cost in doing so make it impractical. As a compromise solution, some antennas are typically manufactured to design specifications at a particular frequency relative to a predetermined frequency range. For example, CB antennas are often factory tuned to a frequency at or below the CB frequency range of 26.95 to 27.405 MHz in order to allow tuning after installation by shortening the electrical length of the antenna. Tuning, in this sense, means causing the antenna to resonate at a desired frequency.
One problem with a pretuned antenna is that the ground plane upon which the antenna is tuned at the factory usually does not accurately reflect the actual ground plane presented by, the vehicle to which the antenna is ultimately installed. As a result, there may be significant signal degradation in the installation away from the pretuned frequency, even to the extent that an antenna cannot be tuned to a desired frequency on a vehicle. In other words, for example, a pretuned CB antenna installed on a vehicle may resonate completely outside the CB frequency range. The prior art has presented a solution to this additional problem by providing tunable antennas whereby after installation, an individual antenna can be tuned to the specific ground plane upon which it is installed. It is known to tune an antenna by changing its electrical length. Available techniques include physically shortening the antenna, e.g., by cutting the end off an antenna, and changing the inductance of a top load, e.g., by moving a slide or a screw in or out of the end of an antenna.
Yet there remains a problem in that there are many models of antennas needed to service all existing models of commercial vehicles at desired resonant frequencies. For example, in North America, there are approximately 40 different models of trucks, each presenting a significantly different ground plane for antennas. Moreover, many new models have fiberglass cabs, which further complicate the problems of providing a properly tuned antenna for vehicles. One can conceive, theoretically, a single tunable antenna that would accommodate the range of different electrical lengths needed for the antenna to resonate at a given frequency regardless of the vehicle on which it is installed. As a practical matter, however, especially for CB antennas, there is no single tunable antenna that can provide enough tuning range to handle all field needs for tuning the antennas to a resonant frequency on all available truck models. One of the principal reasons is that the physical requirements for such an antenna would weaken the antenna or otherwise render it impractical for use in the field. Consequently, especially in the aftermarket for CB antennas, manufacturers and sellers must provide many different tunable antennas to accommodate all of the needs. Moreover, there is also the continuing problem that tuning antennas in the field requires specialized knowledge that may not be available to a user for properly tuning an antenna after installation.
These problems are solved by the present invention of a tunable antenna system comprising an antenna, a tuning mechanism to adjust the electrical length of the antenna, and a key that relates points of adjustment in the tuning mechanism to a data set. Each datum of the data set represents a predetermined installation of the antenna, each installation requiring that the antenna be tuned to a predetermined resonant frequency. Thus, when the antenna is installed in a given installation, it can be tuned to the predetermined resonant frequency by application of the key to the tuning mechanism without specialized knowledge or specialized equipment.
In one embodiment, the key comprises indicia on the antenna. The tuning mechanism can be one comprising two components, one of which moves relative to the other, and the indicia can be on one of the components.
In another embodiment, the key is separate from the antenna. The key can be a guide, or the key can be a gauge corresponding to an installation, that when applied to the antenna will identify the corresponding point of adjustment. The key itself can be on the antenna, such as, for example, when the data set comprises indicia on the antenna.
A method of tuning an antenna in a such tunable antenna system comprises the steps of consulting the key, identifying a point of adjustment corresponding to an installation, and adjusting the tuning mechanism to the point of adjustment. Preferably, the predetermined resonant frequency is in the range of 26.95 to 27.405 MHz.
In another aspect of the invention, a tunable antenna system comprises a set of tunable antennas and a data set. Each antenna of the set is tunable within a range of electrical lengths, and each datum of the data set represents an installation of the antenna. The ranges of electrical lengths are sufficient to provide resonant frequencies for all installations in the data set. The number of ranges is less than the number of installations. Thus, fewer antennas are needed than installations possible to provide a tuned antenna for each installation.
Preferably, the set of installations comprises different models of vehicles, such as trucks. Also, preferably, the resonant frequency is in the range of 26.95 to 27.405 MHz.